1. Field of the Invention
The present invention relates to a control system for use in supplying conditioned air to an enclosure. More particularly, the present invention relates to a control system for co-ordinately synchronizing a heat pump and a furnace to provide conditioned air to an enclosure.
2. Description of the Prior Art
It has been determined that a heat pump is capable of supplying sufficient quantities of heat energy to meet many residential and commerical heating applications even in northern climates. The use of a heat pump to transfer heat energy from an area where loss of heat is not important, such as the outdoor ambient, to an area where the heat energy is required is a very efficient method of heating an enclosure under the appropriate circumstances. Many heat pumps commercially available are capable of transferring up to two or three times the amount of heat energy from one area to another as would be generated using an equivalent amount of electricity for electrical resistance heating. The heat pump having a high co-efficient of performance may be more efficient than a fuel fired furnace under appropriate heating conditions and with proper use resulting in overall energy usage savings.
Heat pumps are, however, limited in overall application since for a heat pump to operate it must be capable of removing heat energy from one area and transferring that heat energy to the area or enclosure to be heated. Heat pumps of modern day design are capable of performing this operation at temperatures well below 0.degree. F. while performing more efficiently than electrical resistance heating. However, when operating at extremely cold temperatures the heat pump is much less efficient and transfers a reduced amount of heat energy. Under these conditions, it may be appropriate to operate a fossil fuel fired furnace which would be more efficient and would be capable of supplying additional heat energy as may be needed to condition the enclosure.
Heat pumps also have the disadvantage that when the refrigerant in the outdoor coil is being evaporated to absorb heat from the ambient air, the air adjacent to the coil is cooled below the freezing point and as it is cooled, the moisture in the air is precipitated onto the outdoor coil surface resulting in frost or ice buildup thereon. The frost buildup becomes an insulating layer further decreasing the ability of the heat pump to transfer heat energy.
It has been found that below certain outdoor temperatures it is both economical and advantageous to use conventional furnace type or boiler heating to supply heat energy to an enclosure. This may include the use of electrical resistance heat or conventional gas, oil or coal fired furnace or the use of a boiler fired by any one of these fuels. The point at which it is desirable to switch from the use of the heat pump to the use of the alternate heating source is called the balance point. This point may be chosen either based on the economics of operating the heat pump versus the furnace or may be chosen on the basis of the capability of the heat pump for supplying sufficient heat energy to maintain the temperature of the enclosure.
Many heat pumps are combined with existing gas or oil furnaces in a residential application to provide an improved conditioning system. Many homes had gas or oil furnaces installed as-original equipment. To provide air conditioning to these homes a refrigeration circuit including indoor and outdoor coils is typically arranged with the indoor coil located in the duct work between the enclosure and the furnace. In lieu of such an air conditioning system it is a simple matter to install a heat pump in place of the air conditioner such that not only will cooling be provided during the cooling season but that heating will be available from the heat pump when desired. By utilizing the heat pump it is possible to obtain efficiencies available by utilizing the heat pump when it is more efficient to operate the heat pump and to utilize the existing furnace when it is more efficient to operate the furnace. In addition, the availability of the furnace provides a source of economical heat energy to supply to the enclosure during defrost of the heat pump to further provide an economical combined system.
Control systems have become commercially available for integrating the operation of a heat pump and a furnace. The herein described method and apparatus specifically concerns the integration of multiple relay means to provide for fail safe operation should a component of the system fail. A heating lockout relay is utilized to prevent operation of the furnace if heat pump operation in the cooling mode is desired. The heating lockout relay is also utilized during the defrost mode when the heat pump is operating and includes contacts normally open preventing furnace boiler operation which close to provide for furnace boiler operation during defrost conditions. The utilization of the heating lockout relay in this application provides for fail safe operation such that should the defrost relay fail, a furnace boiler relay will not be energized unless the heating lockout relay is likewise energized. Additionally, a blower pump relay may be energized when the heating lockout relay is energized but not when the furnace boiler relay is energized such that a control system is provided for allowing the boiler pump relay to be energized when the heat pump is operated but not when the furnace boiler is operated. When the furnace boiler is operated separate circuitry of the furnace boiler is utilized to control fan or pump operation. This system combination allows for multiple fan speed operation and for delays in fan operation when switching between the heat pump mode of operation and the furnace mode of operation.
An interlock relay is also utilized having interlock relay contacts which, when closed, act to maintain the interlock relay energized such that a circuit is maintained through an outdoor thermostat holding the interlock relay energized until both the first and second stages of heating are satisfied. The interlock relay further has contacts for locking out a high temperature switch which would prevent heat pump operation during furnace operation. Additionally, the interlock relay has contacts arranged to connect the furnace boiler relay to a power source to energize the furnace or boiler upon the interlock relay being energized.
Hence, as may be further seen herein, the combination of relays provide for safe operation of an integrated system such that should various components fail operation will still be maintained in a safe and orderly manner throughout the control system.